The invention relates to a sheet feeder for supplying a conveying device with folded signatures, comprising a gripper drum with at least one gripper for removing the signatures individually from a stack. In such a device, a peripherally arranged stopping device on the gripper drum includes a stop element against which the signatures can be aligned, respectively, with the fold forward. An opening mechanism opens the individual signatures aligned against the stopping device and deposits the signatures on the conveying device while also reversing their direction.
Sheet feeders of this type have long been used, for example in gathering and wire-stitching machines. These sheet feeders use a gripper drum to pull folded signatures individually from a stack, to open the signatures and deposit them, for example on a gathering chain. Sheet feeders of this type must meet the requirement of ensuring a trouble-free deposit of the signatures, even if different formats and different types of paper are used.
One essential parameter which limits the production speed is the speed at which the signature impacts the stop element. The impact causes a compressing and buckling of the signatures, especially with thin signatures, or causes the signatures to rebound from the stop element, which causes problems when the signatures are opened with the aid of the opening drums. This problem has long been known and several solutions have already been proposed.
German patent document DE-A-30 35 497 discloses a sheet feeder of the aforementioned type, which is embodied with a movable stop element. The stop element respectively takes over the signatures with a synchronous movement and then slows the signatures down, which is designed to prevent a compressing of the signatures that arrive at high speed at the stop element.
German patent document DE-A-197 38 920 discloses a sheet feeder having a belt arranged upstream of the stop element, which forms a wedge-shaped intake opening for the signatures. The goal is to achieve a stabilization of the signatures during the impact with the end stop by using the friction between the signatures and the belt.
European patent document EP-A-0 716 995 discloses a sheet feeder, for which a guide arrangement that is connected to a stop element for signatures is automatically adjusted and displaced by the supplied signature and for which the stop element itself is made of rubber or a rubber mixture that dampens the impact of the signature.
Especially with heavy signatures, it is difficult even with the aforementioned, proposed devices to sufficiently reduce the kinetic energy at high speeds when the signatures impact with the stop elements, to prevent excessive deformations that would interfere with the further processing.